This invention relates to a process for the working-up of hydrolysis residues which are produced during the hydrolysis of solids-containing polysilane sludges from organochlorosilane synthesis by the direct process. The present invention relates, in particular, to the working-up of hydrolysis residues from methyl chlorosilane synthesis.
Organochlorosilanes and, in particular, the methyl chlorosilanes serve as starting materials for the production of silicones which are widely used, for example, as rubbers, crack sealants, oils, building protecting agents etc. Dimethyl dichlorosilane, which is obtained in a high yield if the direct reaction of silicon with methyl chloride is catalyzed by copper or copper compounds, is particularly desirable during the production of methyl chlorosilanes. The process is basically described in U.S. Pat. No. 2,380,995. Methyl chlorosilanes are produced on an industrial scale world-wide by this process, the reaction normally being carried out in continuously operating fluidized bed reactors.
It is known that further silanes corresponding to the general formula Me.sub.x SiCl.sub.4-x, x=0 to 4, Me.sub.x HSiCl.sub.3-x, x=0 to 2, (Me=CH.sub.3 -) as well as dimeric silanes Me.sub.x Si.sub.2 Cl.sub.6-x, x=2 to 6, siloxanes and silylmethylenes are formed during direct synthesis in addition to dimethyl dichlorosilane.
The finest contents of silicon, catalyst and reacted contact mass are carried off continuously together with the reaction product, the crude silane mixture, and unreacted methyl chloride, when carrying out the direct process in fluidized bed reactors. These dust contents are frequently collected together with the highest boiling reaction products (Kp.sub.760 &gt;160.degree. C.) in a so-called sludge vessel with an associated washing tower. The temperature of the vessel in which the conventional excess pressure of from 1.5 to 10 bar prevails is generally adjusted such that the mixture of solids and condensed fractions is kept sufficiently thinly liquid to allow discharge from this vessel.
According to German Patent No. 2,362,494, the content of the sludge vessel may be depressurized into a stirrerequipped container being under normal pressure and the still-distillable fractions may then be separated from the mixture by heating. The content of this vessel is then generally subjected to hydrolysis.
Actual hydrolysis may be carried out in a waste-pipe in the manner described in DE-PS No. 2,362,494. The disadvantage resides in the fact that hydrolysis is often incomplete owing to the short contact times and that large quantities of water are used.
DE-OS No. 3,005,743 describes a hydrolysis process in which the unpleasant property of adhesion of the hydrolysates is avoided by addition of mineral oil. However, since the hydrolysis are considered to be worthless and have to be dumped, the additional organic content is disadvantageous.
EP No. 0,089,784 teaches that the problem of adhesion may be overcome by observing a minimum chlorine content, but this object is difficult to achieve industrially.
In all the above-mentioned processes, a hydrochloric acid suspension is formed in which the fairly solid hydrolysate is considered to be worthless and has to be dumped. However, these hydrolysates are not without problems as they usually contain from 2 to 10% of predominantly metallic copper which may be partially eluted from the dumped hydrolysis and may therefore represent a pollution hazard for ground water. Furthermore, most hydrolysates obtained are sensitive to oxidation and, in some cases, they even have a tendency for self-ignition, and this prevents dumping according to environmental regulations.
Up to 100 g/l of HCl which are dissolved in the hydrolysis water are obtained by hydrolysis of the Si-Cl-functions. Furthermore, the hydrolysis suspensions which are not further treated may evolve hydrogen, and this is undesirable and may even be dangerous.
It is known from German Patent No. 901,889 that exhausted silicon- and copper-containing contact compositions obtained from the reactor during the production of organochlorosilane may be worked-up by treatment in water or dilute hydrochloride acid with gaseous chlorine. The unchanged elemental silicon is separated from the copper-containing solution after chlorination and the copper-II-chloride is converted into copper-I-chloride by reducing agents, is crystallized out and is re-used as catalyst in silane synthesis.
This process is unsuitable for the hydrolysates under consideration here for two reasons:
Owing to the polysilane content in the material discharged from the sludge vessel, the hydrolysis suspension contains considerable proportions of compounds which are capable of chlorination, and this may lead to waste water problems.
On the other hand, copper cannot be separated quantitatively by crystallization from copper-I-chloride and, furthermore, other metals which are not precipitable as sparingly soluble chloride obviously also remain in solution.